Process for the hardening of steel



Patented July 8, 1941 PROCESS FOR. THE HARDENING OF STEEL Carl Albrecht and Klaus Bonath, Kronberg in Taunus, Germany, assignors to Deutsche Goldund Silber-Scheideanstalt, Frankfort-on-the- Main, Germany, a corporation of Germany No Drawing. Application May 1, 1940, Serial No.

332,844. In Germany May 16, 1939 (Cl. 148-15) i 4 Claims.

Object of our invention is a process for hardening all sorts of steel, especially tool steel, tempered steel and carburized steel. Hitherto, these steels have been subjected to a heat treatment in cyanide containing bath with subsequent quenching in a salt bath of low temperature. To prevent any harmful decarburization in the bath the cyanide contents were considerably high. We prefer to use alkali nitrates or mixtures with alkali nitrites as quenching baths.

Now we have made the observation that during the transmission .of the steel pieces from the cyanide containing heating bath to the nitrate or nitrite quenching baths decompositions or even explosions may readily occur, especially if the cyanide content of the bath is high in order to prevent decarburization. It is unavoidable that during the transmission of the steel objects from the heating bath to the quenching bath quantities of salt adhering thereto will come into the nitrate bath. This leads to explosions and decompositions, a fact which hampered the utilization of the process on a larger commercial scale.

On the other hand experiments which were treatment baths in order to avoid the presence of cyanide wholly or partly have shown that in case during the heat treatment undesired decarburization will readily occur.

Now we have found that under observation of definite conditions it is possible to carry out a heat treatment in a cyanide containing bath as well as a quenching in a nitriteor nitrate bath without any occurrence of the above mentioned disadvantages.

According to our invention all sorts of steel may be heated in improved heating baths which contain mainly compounds of earth alkali and alkali metals and alkali cyanide in quantities of 1-10%, preferably 35%. The steel objects are heated to temperatures of about ZOO-1000 C. and then quenched in a bath containing nitrates and/or nitrites of alkali metals at a temperature of about 150-500 C., preferably at about 200 C.

The heating bath may also contain, for instance, halides and/or oxides or carbonates of the earth alkali or alkali metals. Excellent yields have been observed with a heating bath of the following composition:

Parts NaCN 3-5 NaCl -30 BaCl -60 KCl made to chang the composition of these heat As quenching baths we prefer to use the follow- I ing composition:

NaNO; about 50 KNO3 50 Sometimes we have found it advantageous to add small quantities of chromates to the quenching baths, to stabilize the baths and to preserve the containers.

What we claim is:

1. A process for the hardening of steel which comprises heating steel articles in a fused salt bath composed essentially of compounds of alkali and alkaline earth metals togetherwith 1 to 10% by weight of alkali metal cyanide at a temperature of 700 to 1000 C. and quenching the so heated articles in a fused salt bath containing at least one alkali metal nitrate at a temperature of about 150 to 500 C.

2. A process for the hardening of steel which comprises heating steel articles in a fused salt bath composed essentially of compounds of alkali and alkalin 'earth metals together with 3 to 5% by weight of alkali metal cyanide at a temperature of 700 to 1000 C. and quenching the so heated articles in a fused salt bath containing at least one alkali metal nitrate at a temperature of about 200 C. v

3. A process for the hardening of steel which comprises heating steel articles in a fused salt bath which contains v3 to 5 parts by weight of sodium cyanide, 30 to parts by weight of barlum chloride, 10 to 30 parts by weight of sodium chloride and 20 to 40 parts by weight of potassium chloride at. a temperature of to 1000 C. and quenching the so heated articles in a fused salt bath containing substantially equal parts by weight of sodium nitrate and potassium nitrate at a temperature of about to 500 C.

4. A process for the hardening of steel which comprises heating steel articles in a fused salt bath which contains 3 to 5 parts by weight of sodium cyanide, 30 to 60 parts by weight of barium chloride, 10 to 30'parts by'weight of sodium chloride and 20 to 40 parts by weight ofp'otassium chloride at a temperature of 700 to 1000 C. and quenching the so heated articles in a fused salt bath containing substantially equal parts by weight of sodium nitrate and potassium nitrate at a temperature of about 200 C.

CARL ALBRECHT. KLAUS .BONATH. 

